Metal sheet press-bending machine

ABSTRACT

A press-bending machine to bend metal sheets, having improved features in the respective press-bending with measuring and control system operating on at least four points of the bending angle, of the type in which the press-bending machine has: an upper vertically reciprocal elongated bending punch; a lower static elongated bending matrix with at least a longitudinal bending groove; feeler means to measure the respective bending movement of the metal sheet in bending inside said bending groove, to control by data process logic unit the bending parameters of bending process in said bending machine, said feeler means operating with at least four bending detection points, characterized in that all said detecting points are conceived in such a way to be divided in two sets of bend detecting points, one to one side and one to the other side and in a symmetrical way divided in number and position respective to the vertical plane passing along the respective sheet bending line corresponding with the bending corner of the resulting bent sheet.

TECHNICAL FIELD

This invention has for object a press-bending machine having improvedfeatures in the respetive press-bending with measuring and controlsystem operating on at least four points of the bending angle. Theinnovation finds particular even if not exclusive application in thecontrolled sheet deformation on press-bend working.

BACKGROUND ART

Bending presses are known. They find wide use in the metal andmechanical industry, and in particular in the bending of metal sheets,for example for obtaining some differently shaped longitudinal sections,sometimes with the possibility of being taken up and, each of them,subjected again to a bending-pressing cycle. As a rule, it is possibleto notice that a bending cycle consists essentially of the verticaldescent of a tool up to touching the underlying metal sheet resting onthe bench, in the carrying out of the bend, and then, at the end, ingoing back (lifting) up to reaching a starting position.

For carrying out the previous phases, the machine is made up of twoparts, respectively a dynamic upper one (movable upper part), and astatic lower one, making up the underside of the machine placed on theperpendicular of the dynamic part.

Regarding the dynamic part, in the execution of a bending cycle, themovable bending tool (elongated punch), made up of a blade differentlyshaped, also of the interchangeable type, effects exclusively a verticalto and fro movement, ensured by at least one oleodynamic cylinder, whichdetermines the descent of an upper cross-piece which supportslongitudinally said elongated punch, said punch operating towards alower cross-piece that supports an interchangeable elongated matrix,followed by the eventual stop and lifting in the starting position.

In existing solutions, some drawbacks are noticeable. These, in general,concern the inaccuracy of the bending angle and in any way relate to anobjective difficulty of predetermination and measuring of the bendingangle. The traditional system provides that, giving a known total heightof the matrix and depth of the groove in the matrix bending area, andthe thickness of the sheet, the punch lowers to touch the sheet, then tofurther lower of a prestablished height to reach the required bendingangle.

In the machines with numerical control, the punch descent is calculatedmathematically on the base of some parameters set by the operator, andconsequently, the machine is prearranged for executing the programmedangle. However the result is not always optimal, as such technique, manytimes leads to obtaining angles with some errors even if limited ones.This happens because of the presence of different factors, for example,the thickness of the sheets which is not constant, where even theincidence of few hundredths of a millimeter negatively influences theworking.

For other reasons, because said predetermined theoretic calculation,such system does not offer the possibility of really checking theresult, at the moment of bending, with the risk of endangering theproductive process.

An additional factor having considerable character, relates to thenatural elastic return of the material, which is calculatedhypothetically and therefore in so far as it can be reliable, it may getclose to the desired result, but it will never be considered as a realdata.

Finally, besides the faultiness of the product, it is necessary toconsider that the desired result is never attainable at the firstbending cycle, that is at the first press-bending, but generally, asecond press-bending phase is always necessary that intervenes tocorrect the first result.

With the aim of solving the problems pointed out, some complex pressmachines were conceived, which use a bending matrix supplied with anadjustable bottom, allowing to obtain a bending angle more precise thanthat of the traditional systems.

From a practical point of view, said matrix provides two surfacescoplanar and movable on horizontal plane defining in an intermediateposition a longitudinal groove whose bottom may be eventually modifiedin height.

Such groove determines the momentary bending angle by means of therelative position of both the support surfaces on the side of thegroove, which delimit its opening, and the bottom of the same groove.

Also in this hypothesis, however, persists a certain inaccuracy, one ofwhose causes is ascribable to the phenomenon of the elastic return ofthe sheet, a condition which occurs at the moment which follows thepiece discharge, altering the bending angle originally determined andtheoretically calculated.

Consequently, is necessary to proceed at first with some working tests,and, before starting the definitive bending production cycle, to carryout the due corrections on the numerical control, intervening on thepushing action of the punch and eventually on the position of the matrixbottom.

All this, besides requiring the intervention of specialised personnel,involves the machine stop and in conclusion, a considerable loss ofuseful time unavoidably influencing the relative production costs.

European Patent 340 167 (Hammerle) document, proposes a bending processaccording to a given nominal angle with the aid of a bending equipmentmade up of a punch and of a matrix, which is supplied with an adjustablebottom according to the angle to be formed.

The text points out that the process consists in that it provides:

in a first phase, the adjustment in height of the matrix bottom occurson the basis of the first angle to be obtained, which is a little widerin respect to the given nominal angle, where the sheet is bent on thebasis of this first angle by the lowering of the punch up to the matrixbottom;

in a second phase, the section is discharged, so that a return of thesame in the stretched position occurs;

in a third phase the measured angle deriving from the returned andstretched section, is compared with the first angle and the position ofthe matrix bottom is adjusted with a value that corresponds to thenominal angle minus the difference between the angle measured on thereleased section and the first angle;

in a fourth phase the bent sheet is completely pressed by the punchagain charged against the matrix bottom, which will take a correctposition in height.

However even this solution is not free from drawbacks.

First of all, it appears as an extremely complex machine, not flexibleand somewhat oversized, which needs a constant and accurate maintenanceand setting-up, predominantly feasible by highly specialised personnel.

The consequence is, for the reference market, some high costs, above allrelated to the purchase and management of the machine itself. From aqualitative view-point, finally, said solution does not allow to obtaina sheet bending with the round corner on the extrados, therefore optimalfor the next processing.

And in fact, it can be noticed that in the bending phase, by using athird dynamic point as mechanical element provided on the matrix bottom,the sheet in logic correspondence tends to be deformed, getting flat,practically becoming squashed, even if slightly, mainly incorrespondence of the extrados of the bending angle.

A proposal, that can help to solve part of the problems previouslypointed out, was put on the market by the Belgian Company LVD with thesystem named Easy-Form®.

Said system, consists in providing a movable arm, placed on the matrixside, which, supported by two articulations, and in bending phase,places a sensor means in contact with one of two wings of the divergingsheet.

Said sensor means, is coaxially movable in respect to said arm, andprovides a measured data to the control logical unit of the machine. Inthis hypothesis, there are therefore three measuring points for givingparameters to the machine, two of which known, made up by theintersection corners of the plane with the matrix bending groove, andone variable and detectable by the oscillation with followingpositioning of said movable arm.

However, this is just because of the improper side position of the thirdpoint, dynamic, in respect to the matrix for the measuring of thebending angle, that a satisfactory precision is not allowed as, becauseof the natural characteristics of the material, a data unlike anddifferent from the real data objectively concerning the bending anglewould result. =p Finalised to radically solving the problems of thesolutions previously described, the applicant with the Italian PatentApplication TV97A000039 (Gasparini), proposes a press-bending process ofthe metal sheet by a direct measuring system, in which the following isprovided:

the advancement of the metal sheet on the working bench, up tointersecting the vertical axis plane of the upper punch supported by anupper cross-piece, towards the underlying matrix supported by a lowercross-piece; and in which on the back of the sheet foil feel a feelermeans, passing into the matrix and connecting each with a respectivemeasuring group, each of which communicates with a data processinglogical unit that controls said press-bending machine;

therefore, after carrying out the descent phase of the punch, towardsthe underlying matrix, and then press-bending the sheet and determines acorresponding displacement along the vertical axis of said feeler means,which, being in co-operation with reading means of a correspondingmeasuring group, communicate to the data process unit the data relativeto the bending stroke;

at the end, in proceeding with the reascent of the punch, by carryingout at the same time the reset of said feeler means to their originalcondition;

and again in which, detecting in the first phase through said feelermeans, permanently in contact with the sheet foil, a bending angledifferent in respect to the preset nominal one, said data process unitensures the consent to the press-bending machine, not discharging theproduct so obtained, to carry out at least a second descent phase of thepunch, towards the underlying matrix up to passing again on the samebending angle, to then proceed with the discharging of the product.

In relation to said process, it is finally an opinion of the applicant,that the working and above all the measuring phase of the bending anglemay be further optimized, above all with regard to the precision and thereading times of the bending angle obtained, not excluding thepossibility of intervening for the correction of the elastic return ofan already pressed-bent sheet.

A recent system for the measuring of the bending angle characterised bythe trade-mark ACB®, and disclosed in DE 195 21 369, was proposed by theFirm TRUMPF and concerned a product named TrumaBend series V. It, inpractice, consists in providing on the inside of the upper bending tool(punch), two feeler discs, with different diameters. During the bendingprocess the discs are self-centred measuring four contact points on theinternal side of the bend, and consequently, on the basis of thedistance of the centres of the discs, the system allows to calculate theeffective angle, said discs being changed with independtly movable pinsplaced on either side of the punch.

The main drawback, which can be ascribed to the above mentionedsolution, consists essentially of the fact that it is not possible tointervene on the bendings with such measuring system, in which thesheet, relatively to the angle obtainable on the internal side of thebending, is wider than 90° to 10°.

Said system, additionally, obliges to maintain the sheet edges somewhatwide, reducing the use possibilities of the different matrixes, with aconsequent lower flexibility of the press-bending machine.

Finally, said system predetermines the bending angle by means of somecalibration matrixes, and consequently, on one side involves alimitation of the bending measuring and on the other does not allow arapid obtainment of the bending desired, above all in consideration ofthe fact that it needs a complex setting. U.S. Pat. No. 4,489,586discloses a bending feeler means passing through the respective punch.U.S. Pat. No. 4,131,008 discloses a “V” shaped axial feeler meanspassing through the V die matrix.

In relation to the two latter techniques for the measuring of thebending angle, it is the opinion of the applicant, that the working andabove all the measuring phase of the bending angle may be furtheroptimised, above all with regard to the precision and the reading timesof the bending angle obtained, not excluding the possibility ofintervening for the correction of the elastic return of an alreadypressed-bent sheet.

These and other problems are solved with this innovation according tothe characteristics as in the included claims, by a press-bendingmachine to bend metal sheets, with measuring and control system of thebending angle, of the type in which the press-bending machine has:

an upper vertically reciprocable elongated bending punch;

a lower static elongated bending matrix with al least a longitudinbalbending groove;

feeler means, having bending detecting points, to measure the respectivebending movement of the metal sheet in bending, on said bending groove,to control and command by data process logic unit the bending parametersof bending process in said bending machine, wherein all said bendingdetecting points are conceived in such a way to be divided by animmaginary vertical plane passing on the bending corner line of thebending metal sheet, in two sets of bending detecting points, one set ofdetecting points to one side and one set of detecting points to theother side, wherein said bending detecting points are realized by avertically elastically movable feeler means, moving on the verticalplane crossing the corner of the “V” groove, independently of respectivebending punch movement, characterized in that: along said bending grooveof said matrix, said feeler means is made up of a couple of forksmutually interacting, the one inside or adjacent in respect to theother, in such a way that the median axis of both said forks coincidingwith the axis of said punch, and in which said forks are elasticallyyielding and are downwardly connected with a relative positiontransducer communicating with a data process logical unit that managessaid pressing-bending machine.

Advantageously the feeler is substantially a fork and a bendingoperation is as follows:

the advancement on the working bench of at least one metal sheet, up tointersecting the descent vertical axial plane of said punch supported bya reciprocally movable upper cross-piece, toward the underlying matrixsupported by a lower cross-piece; and in which on the sheet rested onthe matrix, feel at least one feeler means made up of said feelermeasuring fork interacting with said position transducer, communicatingwith a data processing logical unit that controls said press-bendingmachine;

therefore, in carrying out a first descent phase of the measuring punchsupporting upper cross-piece, towards the underlying matrix supported bya lower cross-piece, pressing-bending the sheet foil and determining acorresponding displacement along the vertical axis of said feeler means,which, interacting with said position transducer, communicates to saiddata process unit the data relative to the bending stroke;

at the end, in the carrying out of at least one partial re-ascent of thepunch, carrying out at the same time the reset of the feeler means in anoriginal starting condition;

and again in which, detecting in the first phase through said feelermeans, a bending angle different in respect to the preset nominal one,said processing unit ensures the consent to the press-bending machine,not discharging the product so obtained, for carrying out at least onesecond descent phase of the punch, towards the underlying matrix up toresting again on the same bending angle, for then proceeding with theproduct discharging.

In one solution the use of a feeler measuring fork realizessubstantially two feeling angled points wherein the other two anglepoints is realized in the borders (corners) of the elongated groove ofthe matrix.

Better performance is mainly due to the improved measuring system of thebending angle, which besides being extremely precise, is always suppliedin real time, allowing to intervene in a determinant way for the errorcorrection, up to obtaining the nominal bending angle with the dueprecision.

In this way, the reading during the bending is more precise, thusavoiding bending errors.

Using the fork-like shaped feeler means, the bending angle is measured,not as if this were an ideal geometric figure defined by three points,but viceversa considering the real inclination of the two specularplanes each interested by two measuring points one of which is known andthe other is variable, definitely overcoming the errors caused by thethickness of the sheet, by the material composition, and of the thinningcaused by the sheet stretching in correspondence of the edge.

The consequence is that, once the machine is set for the obtainment of acertain bending angle, it is possible to bend another sheet of anymaterial and thickness, provided that it is compatible with the width ofthe bending groove, without modifying the set programming and withoutcarrying out tests.

It must be additionally considered that the use of such feeler meansallows a radical simplification of the press-bending machine, with, onone side an important reduction of the manufacturing costs and on theother a fair containing of the encumbrances of the respective devices.

As a consequence thereof, it requires a minimum maintenance, easilypossible by suitable personnel and with short stops of the productivecycle.

This system has the further advantage of simplifying the execution ofthe control software as this means a measuring of only a lineardisplacement of the feeler fork. Finally, the system for maintaining thepressed-bent sheet in place is particularly interesting and innovative,for allowing the execution of a second bending phase, suitable tocorrect the elastic return of the sheet measured after a first bendingphase.

Said system, substantially, avoids the pressed-bent sheet, from beingsubjected to displacements, also accidental, in respect to the matrixand this until the punch has not again touched the bending groove forthe correction of the error detected in respect to the nominal data ofthe preset angle.

Advantageously:

on the back of the metal sheet rested on the matrix, permanently feel atleast one feeler means, provided along the corner axis of the bendinggroove of said matrix, said feeler means being made up of a couple offorks mutually interacting, the one inside or adjacent in respect to theother, in such a way that the median axis of both the forks coincideswith the axis of the punch, and in which said elastically yielding forksare connected with a relative position transducer communicating with adata process logical unit that manages said press-bending machine; Withthis solution the measuring of the bending angle is made possible, onfour dynamic measuring points or bending detecting points, with themaximum precision possible without any limitation relative to the angleto be obtained.

This solution does not need the execution of a perfect groove in thematrix, as the aforesaid measuring system is independent of anyreference on the same.

The measuring system is independent from the eventual elasticdeformation of the matrix, in the bending execution phase.

The bending time can be reduced, accelerating the whole productiveprocess. Such result is more evidently considered the most effectivemeasuring system of the bending angle, which other than being extremelyprecise, always provides the data in real time, allowing to intervene ina resolutory way for the error correction, up to obtainin, with dueprecision, the nominal bending angle.

As the reading of the angle during the bending occurs on the same sideof the sheet, in the first case on the lower surface and in the secondcase in the upper surface of the sheet, errors due to the change ofthickness of the sheet are avoided.

Additionally, it is found that by using one of the above mentionedfeeler means, the bending angle is measured considering the realinclination of the two specular planes of the sheet, each concerned, onthe intrados or on the extrados by the couples of measuring points,overcoming in a definitive way the errors caused by other factors egyielding (kind of the material), and by the thinning caused by thestretching in correspondence of the edge.

The result is that, once set the machine for the obtainment of a certainbending angle, it is possible to bend another sheet of any material andthickness, provided that it is compatible with the width of the bendingmatrix groove and respective punch, without modifying the programmingset and without carrying out tests and this also with small bendings.

These, and other advantages will appear from the following descriptionof same solutions with the aid of schematic drawings, whose details arenot to be considered as limitative but only illustrative.

FIG. 1, shows a view, in detail and with reference to the previousFigure, of the four bend detecting points detected by the measuringdevice, from which are obtained respective data necessary in thedetermining of the actual bending angle.

FIG. 2, shows a front view of a press-bending machine, in which,relative to the lower cross-piece, some devices for the bending anglemeasuring are pointed out.

FIGS. 3 and 4 show the solution of the fork feeler means using a doublefork as for this invention in the two main phases, before bending thesheet and after bending it.

FIG. 3, shows a view in detail of one of the phases of the sheet bendingprocess, seen in correspondence of the punch which rests on the matrix,determining the displacement, along the vertical axis, of a measuringdevice including a feeler means consisting of a double fork.

FIG. 4, shows a view of a subsequent phase of the process for theworking of the sheet foil found in FIG. 11, seen in correspondence tothe punch that passes on the matrix, determining the displacement, alongthe vertical axis, of a measuring device including feeler means made upof a double fork.

Considering the FIGS. 1 to 2, it can be seen that a press-bendingmachine (A), is made up of an upper and lower part, the firstessentially dynamic in respect to the second one, static.

Of the first one, is part an upper cross-piece (1), vertically movablealong the vertical axis (Y′-Y″) in respect to the frame of thepressing-bending machine, on whose lower end is provided associated,longitudinally, a tool of the interchangeable type, making up the punch(2).

The press-bending machine (A), provides at the ends, a cylinder means(3, 3′) for each side, which determines the descent and lifting movementalong the axis (Y′-Y″), of the upper cross-piece (1) towards theunderlying lower cross-piece (4), which supports a matrix (5) also ofthe interchangeable type.

Said matrix (5) has longitudinally, at least one bending groove (5′)that determines the bending angle “a” of a sheet foil (B) subjected to aworking cycle.

In this case, along the longitudinal groove (5′) of a matrix (5), isprovided at least one measuring area, in this case two of them (r′-r″),placed at the ends of the same one, or near to the sheet ends (B), andmore in detail a right one and a left one (r′-r″).

On the top of said groove (5′), the walls, obtained by the intersectionof the tilted planes with the horizontal plane of the matrix (5), twoopposite corners, realizing a first couple of said angled bend-detectingpoints (6, 7) are obtained, on which corners the sheet (B) in bendingphase works.

The groove (5′), provides on the bottom, logically corresponding to eachof the two areas (r′-r″), axial vertical holes (105), on whose inside itis vertically movable, following a stroke (y¹), a relative nailfork-like feeler (9-106: 108-108′: “Rc”). Said feeler (9-106), incorrespondence to the upper end, is provided with a head (107-107′:“Rt”) both with a fork-like or even with “U”-like shape, suitable totouch with the two angled points and permanently, the back of the sheet(B), while on the other side said feeler (9-106), interacts with arelative measuring group which transmits the data to a data logicprocess unit of the information thus acquired.

In this case, the measuring system of the bending angle “α”, thatconcerns at least the two end areas (r′-r″) uses substantially fourmeasuring points for each of them, respectively, two static ones (RcFIG. 1) which correspond to the centre of the curvature radius of thecorners of the groove (5′-102′) of the matrix (5), and two dynamic ones(Rt FIG. 1), as feeler fork points (107-107′; 108-108′) symmetricalyplaced one to one side and one to the other side of the bending corneraxial line; both operating on the back of the sheet (B).

In this case, the two dynamic points (Rt), will be diametrically opposedin respect to the bending axis (Y′-Y″), where the horizontal distancebetween the centres (Rc) and (Rt) is not always constant (Ce), whereasthe distance (Ci) between said two dinamic feeler points (Rt) isconstant. In substance, we have four feeling points (2×Rc+2×Rt) of whichtwo static detecting points (Rc) and two dynamic detecting points (Rt)that integrated in a movably dynamic feeler device (9) costitutes twofeeler points (Rt), the total remaining four detecting points. Bothcouples of points beng divided symmetrically in position and number toone side and to the other side of said vertical axial plane passing onthe bending corner line (y1) The detecting points result symmetricrespect to the bending axis (Y′-Y″). While the position of the staticdetecting points (Rc) is known to the data processing unit of themachine (A), the exact position of dynamic feeling points of feelerpoints (Rt), is detected by said feeler fork-like shaped (9-106), thatis maintained pressed against the lower surface of the sheet (B), withthe feeler points one on one side and the other to the other side of thebending corner line (Y1 axis). Therefore, knowing the displacement (H₂)between the static couple (Rc) and the dynamic couple (Rt) it ispossible to calculate the bending angle “a” as it corresponds exactly tothe tangent of the radii (Rc) and (Rt).

In a working cycle, when the sheet (B) is placed over the matrix (5),the four bend detection points (Rc, Rt) of each area (r′-r″), are placedperfectly aligned and coplanar.

In this condition, by means of the optical transducer group, theprocessing unit feels the position of the nail-like feeler (9-106), andconsiders it as a “0” value (Index).

On the practical side, it is preferable that the data process unitprovides that the feeler means (9-106) is put in position, touching theback surface of the sheet (B). By carrying out the bending, the sheet(B), curves penetrating toward the groove (5′-102′), pushingconsequently downwards also the fork-nail-like feeler (9-106), whosefork-like shaped ends (107-107′; 108-108′) remains in contact with theback of the sheet (B). Consequently, the program of the processing unit,will have to calculate mathematically, relating to the bending angle,the stroke (Y′-Y″) of the descent of the punch (2-101), in function ofthe fixed distances, measurable between the fork-like shaped ends(107-107′: “Rt”; 108-108′: “Rc”) in contact with the sheet (B), therelative couple of points (Rt, Rc), inside the matrix groove corners(102′), thus establishing the required bending angle parameters.

In this way, it is obtained that the stroke (Y′-Y′) of the descent ofthe punch (2), is the same stroke (y¹) detected by the feeler ensemble(9-106), which, is read by the corresponding transducer.

More in detail, the displacement (Y′-Y′) of the punch (9) is checked, bya first series optical lines up to its contact with the sheet (B), andthen the movement control is carried out by the said optical lines of ameasuring devices of the bending angle.

Initially, in a bending cycle, the upper cross-piece (1) of the pressingmachine (A), descends at high speed carrying the punch (2-101) towardthe matrix (5-103).

Such displacement is electronically controlled, thanks to two lineartransducers (14), placed to the sides of the press machine (A).

Some millimeters from the sheet (B), the punch (2-101), slows and passesto the low speed up to touching the surface of the sheet (B).

It is at this moment that the reading related to the stroke of the punch(2-101), is given in charge to the transducers placed on the bench, andin fact, it is the sheet (B) pressed by the punch (2-101), that pushesthe feelers (9-106), that start the reading mechanisms.

Once an eventual error between the bending angle “a” and the nominalangle is detected, the machine is pre-set for a subsequent anddefinitive bending cycle, which, without discharging the product (B),will be executed with correction parameters compared and obtained by thereading and processing of data collected in the previous phase.

The press-bending machine (A), can include a punch (2-101) suitable toease the keeping in position of the sheet (B) during the bending phase,following a first one, for the removing of the bending angle differencenoticeable because of the elastic return of the same.

In particular in FIGS. 11, 12, that show said pressing-bending sensingdevice activated by the sheet B pushed down by said punch (2-101).

The second one lower part of the press (4) includes an elongated matrix(9-102) that has longitudinally, at least one longitudinal bendinggroove (102′) that determines the bending angle “α” of the metal sheet(B) subjected to a bending cycle.

In this case, along the longitudinal bending groove (102′), at least onemeasuring area, of said bending angle “α” is provided, for example twoof them, placed at the ends of the elongated bending groove (102′), ornear to the sheet ends (B).

In said matrix, on the top of said elongated bending groove (102′), thecorners obtained by the intersection of the tilted planes with thehorizontal plane of the elongated matrix (102) realizes two symmetricalopposite detecting points (103, 104), on which works the sheet back (B)in bending phase.

The bending groove (102′), provides on the bottom, corresponding to eachof the two detecting areas, holes (105), on whose inside a relativefeeler (106) is vertically movable following a stroke (y¹), said feelerbeing realized as an “Y” shape.

Said fork-like shaped feeler means (106), is substantially made up oftwo Y-rods, whose upper ends make up respective fork (106′, 106″),Y-like or U-like shaped, one placed on the inside or aside in respect tothe other, having a different distance between centres and between therelative bend detecting points (107-107′; 108-108′).

More in detail, the fork (106′) has a distance between centers betweenthe respective substantial points (107, 107′) wider than that of thefork (106″), whose substantial points (108, 108′) define a distancebetween centers shorter than the previous one.

In this case, it is found that the median axis passing through saidforks (106′, 106″) corresponds to the axis (y1) of the stroke of thepunch (101).

With regard to the lower ends of the rods including on the upper parttwo forks (106′, 106″), they are engaged with corresponding elasticallyyielding means (1013, 1013′), in this case made up of compressionhelical springs, and each engaged with a relative position transducergroup.

Purpose of the position transducer group is that of communicating with adata process logical unit of the press-bending machine, giving the datarelative to the different stroke from each single fork (106′, 106″),carried out as a consequence of the pressure perpendicularly exerted bythe elongated punch (101).

In this way, two specular planes can be detected, corresponding to theback (lower surface) of the bent sheet (B), comparing the difference inheight found between the respective substantial points (107, 108) and(107′, 108′).

What is claimed is:
 1. A press-bending machine for bending a metal sheetcomprising: a bending punch; a bending matrix comprising a longitudinalbending groove; a feeler coupled to the bending groove, the feelercomprising bending detecting points, wherein the bending detectingpoints are configured to measure the respective bending movement of themetal sheet during bending, wherein the bending detecting pointscomprise two or more forks, and wherein the forks are positioned in sucha way that the median axis of the forks coincides with the median axisof said punch, and at least one relative position transducer coupled atleast one of the bending detecting points, the relative positiontransducer being configured to determine the bending angle of the metalsheet during use.
 2. The press-bending machine of claim 1 wherein thebending detecting points comprise a first fork and a second fork, eachof the forks comprising a pair of arms, wherein the distance between thearms of the first fork is different than the distance between the armsof the second fork, and wherein the first fork is positioned within thesecond fork such that the arms of the first and second forks arecoplanar.
 3. The press-bending machine of claim 2 wherein the planedefined by the arms of the first and second forks is the same as themedian axis of the punch.
 4. The press-bending machine of claim 1,wherein the forks are coupled to the relative position transducer. 5.The press-bending machine of claim 1, wherein the forks are coupled tothe relative position transducer by an elastic yielding component, andwherein the forks are vertically elastically movable.
 6. Thepress-bending machine of claim 1, wherein the feeler comprises two setsof bending detecting points oriented on opposite sides of the bendingmatrix.
 7. The press bending machine of claim 1, wherein the bendingpunch is coupled to at least one linear transducer, wherein the lineartransducer is configured to control the vertical displacement of thebending punch.
 8. The press bending machine of claim 1, furthercomprising a data process logical unit, wherein the data process logicalunit is configured to control the bending parameters of the pressbending machine during use, wherein the data process logical unit iscoupled to the relative position transducer.